In order to meet the requirements of reducing emissions of exhaust gas and carbon dioxide according to the environmental regulations or the like, hardware modifications have been made on industrial machines and construction machines to employ motorization, an electricity storage system using a regenerative energy, or the like.
Further, in order to meet a demand for an essential technique for improving the properties of a hydraulic oil used in a hydraulic system to achieve a high efficiency, fuel-saving and power-saving hydraulic oils are disclosed (see, for instance, Patent Literature 1).
Since a pressure in hydraulic equipment has become increasingly high from year to year, troubles such as scoring of a hydraulic pump are frequently caused. Accordingly, the ISO standard has employed an FZG scoring test. A typical method for improving an FZG scoring resistance is adding an acid phosphate or an amine salt thereof or an active sulfur compound (see, for instance, Patent Literature 2).